Superabsorbent polymers (SAPs) are well known in the art. They are commonly applied in absorbent articles, such as diapers, training pants, adult incontinence products and feminine care products to increase the absorbent capacity of such products while reducing their overall bulk. The SAPs generally are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight.
Commercial production of SAPs began in Japan in 1978. The early superabsorbent was a cross-linked starch-polyacrylate. Partially neutralized polyacrylic acid eventually replaced earlier superabsorbents in the commercial production of SAPs, and is the primary polymer employed for SAPs today. They generally consist of a partially neutralized lightly cross-linked polymer network, which is hydrophilic and permits swelling of the network once submerged in water or an aqueous solution such as physiological saline. The cross-links between the polymer chains assure that the SAP does not dissolve in water. SAPs are often applied in form of small particles, such as fibers or granules.
After absorption of an aqueous solution, swollen SAP particles become very soft and deform easily. Upon deformation the void spaces between the SAP particles may be blocked, which drastically increases the flow resistance for liquids. This is generally referred to as “gel-blocking”. In gel blocking situations liquid can move through the swollen SAP particles only by diffusion, which is much slower than flow in the interstices between the SAP particles.
One commonly applied way to reduce gel-blocking is to make the particles stiffer, which enables the SAP particles to retain their original shape thus creating or maintaining void spaces between the particles. A well-known method to increase stiffness is to cross-link the carboxyl groups exposed on the surface of the SAP particles. This method is commonly referred to as surface cross-linking.
From European Patent Application EP 0 248 437 A2 a water-soluble peroxide radical initiator as surface cross-linking agent is known. An aqueous solution containing the surface cross-linking agent is applied on the surface of the polymer. The surface cross-linking reaction is achieved by heating to a temperature such that the peroxide radical initiator is decomposed while the polymer is not decomposed.
European Patent Application EP 1 199 327 A2 discloses the use of an oxetane compound and/or an imidazolidinone compound for use as surface cross-linking agent. The surface cross-linking reaction is carried out under heat, wherein the temperature is preferably in the range of 60° C. to 250° C. Alternatively, the surface cross-linking reaction in EP 1 199 327 A2 is achieved by a photo-irradiation treatment, preferably using ultraviolet rays.
In general, the surface cross-linking agent is applied on the surface of the SAP particles. Therefore, the reaction preferably takes place on the surface of the SAP particles, which results in improved cross-linking on the surface while not substantially affecting the core of the particles. Hence, the SAP particles become stiffer and gel-blocking is reduced.
A drawback of the commercial surface cross-linking process described above is that it takes a relatively long time, commonly at least about 30 min. However, the more time is required for the surface cross-linking process, the more surface cross-linking agent will penetrate into the SAP particles, resulting in increased cross-linking inside the particles, which has a negative impact on the capacity of the SAP particles. Therefore, it is desirable to have short process times for surface cross-linking. Furthermore, short process times are also desirable with respect to an overall economic SAP particle manufacturing process.
Another drawback of common surface cross-linking processes is that they take place only under relatively high temperatures, often around 150° C. or above. At these temperatures, not only the surface cross-linker reacts with the carboxyl groups of the polymer, but also other reactions are activated, e.g. anhydride-formation of neighboured carboxyl groups within or between the polymer chains, and dimer cleavage of acrylic acid dimers incorporated in the SAP particles. Those side reactions also affect the core, decreasing the capacity of the SAP particles. In addition, exposure to elevated temperatures can lead to colour degradation of the SAP particles. Therefore, these side reactions are generally undesirable.
SAPs known in the art are typically partially neutralized, e.g. with sodium hydroxide. However, in the processes known in the art, neutralization has to be carefully balanced with the need for surface cross-linking: The surface cross-linking agents known in the art only react with free carboxyl groups comprised by the polymer chains but they are not able to react with a neutralized carboxyl groups. Thus, the carboxyl groups can either be applied for surface cross-linking or for neutralization, but the same carboxyl group cannot be applied to fulfil both tasks. Surface cross-linking agents known in the art generally do not react with chemical groups other than carboxyl groups, e.g. they do not react with aliphatic groups.
In the process of making SAP particles, neutralization of free carboxyl groups typically comes first, before surface cross-linking takes place. Indeed, the neutralization step is often carried out in the very beginning of the process, before the monomers are polymerized and cross-linked to form the SAP. Such a process is named ‘pre-neutralization process’. Alternatively, the SAP can be neutralized in the middle of polymerization or after polymerization (‘post-neutralization’). Furthermore, a combination of these alternatives is also possible.
As the overall number of free carboxyl groups on the outer surface of the SAP particles is limited by the foregoing neutralization, it is very difficult to obtain particles with a high degree of surface cross-linking and hence, a high stiffness to reduce gel-blocking. Furthermore, it is very difficult to obtain SAP particles with evenly distributed surface cross-linking, as the remaining free carboxyl groups are not only few in number but generally also randomly distributed, which sometimes results in SAP particles with regions of rather dense surface cross-linking and regions of sparsely surface cross-linking.
It is therefore an objective of the present invention to provide SAP particles, which have a high degree of surface cross-linking and at the same time allow for a high degree of neutralization.
It is a further objective of the present invention to provide SAP particles with evenly distributed, homogenous surface cross-linking. Moreover, SAP particles should have improved hydrophilicity.
Furthermore, it is an objective of the present invention to provide a method to produce the SAP particles with such properties.
It is a still further objective of the present invention to provide a method for producing SAP particles, wherein the process step of surface cross-linking can be carried out quickly to increase the efficiency of the process.
Moreover, a further objective of the present invention is to provide a process to produce SAP particles, which can be carried out at moderate temperatures in order to reduce undesired side reactions, initiated by elevated temperatures, such as anhydride-formation and dimer cleavage.